Synthetic resins have been used widely, owing to their excellent physical and chemical properties, in a variety of industrial fields such as fibers, films and sheets and other various formed products.
For example, owing to its properties such as transparency and toughness, polypropylene film has been used in a variety of fields, e.g., overlapping wrapping materials for foods, tobacco, cassette tapes and the like, and synthetic paper materials.
For example, polyethylene terephthalate has been used in large quantities for magnetic recording media, capacitors, thermal transfer ribbons, thermal stencil printing master plates, pre-paid cards in film and sheet fields; for rubber-reinforcing fibers for tier cords, V-belts, conveyer belts in fiber fields; and for various clothing materials.
These resin films and sheets are required to have a blocking preventing capability and a slipping property as physical properties needed at the time of handling for forming process and using them as products and fine particles are added to the resin films and sheets so as to make their surfaces properly uneven and provide such physical properties.
Examples of a method for adding the fine particles to the resin films and sheets include: an external addition method for adding inorganic particles such as calcium carbonate, titanium dioxide, silica, talc, kaolin, or zeolite to resins; and an internal precipitation method for precipitating fine particles in resins from residues or the like at the time of resin synthesis like the case of polyethylene terephthalate.
The external addition method is capable of adjusting the degree of the roughness to a certain extent by adjusting the particle size of the inorganic particles and is excellent in controllability of the addition amount and uniform dispersion in resins by adjustment before addition. However, since the affinity between particles and resins is inferior, gaps so-called voids are formed between the resins and particles and there may be caused adverse problems of deterioration of transparency of films and sheets, dissociation of particles from the resins, formation of slight scratches by particles and the like.
Since the internal addition method for precipitating the fine particles gives good affinity between the produced particles and resins, void formation and particle dissociation from the resins scarcely take place and high transparency and low haze can be obtained and thus this method is advantageous to provide physical properties which are difficult to achieve by techniques of the external addition method. However, it is difficult for the method to control the particle diameter of particles to be precipitated and to evenly disperse the particles in the resins; therefore, the method is incapable of providing the resin surface roughness as designed and thus cannot give the anti-blocking property and the slipping property.
In order to solve the above-mentioned problems, in general, precipitated or gel type silica is properly selected and used for a film made of polyolefin such as polypropylene resin or polyethylene resin. Such silica is in the form of an agglomerate basically comprising fine particles, and owing to kneading with a resin in the case of film production process and the stress attributed to resin drawing, the agglomerate is suitable to be properly dispersed in the resin. While suppressing void formation, dissociation of particles, and formation of small scratches following the stress of the resin drawing owing to the dispersion, such silica in the form of the agglomerate can be characterized in that the agglomerate with proper size forms proper film surface roughness after the dispersion.
Further, because of its agglomerate form, the silica contains voids in the particles and thus has a low density and as compared with particles having no voids, the number of the silica particles in the same weight and the same addition amount can be high and accordingly, it is advantageous to provide the properties such as the anti-blocking property and slipping property that are closely related to the number of particles.
Japanese Unexamined Patent Publication Nos. 64-4240 (1989) and 07-196316 (1995) describe study and development of surface treatment agents for providing proper roughness to the surface of resin films, and improving the affinity between inorganic particles and resins enhancing the granular properties such as particle diameter, sharpness and the like of inorganic particles, and propose methods for preventing the formation of voids and dissociation of the particles by carrying out surface treatment of the inorganic particles with the agents.
Further, for example, Japanese Unexamined Patent Publication No. 05-4812 (1993) discloses improvement of the affinity with resins by making the particle surface uneven by binding further smaller spherical silica to spherical silica having a sharp particle size distribution.
Additionally, Japanese Patent No. 3260881 discloses agglomerates (secondary particles) of primary particles of calcium carbonate with excellent particle properties and proposes that the affinity with resins should be improved by limiting the shorter diameter of the primary particles, the longer diameter of the secondary particles and their ratio.
As particles having voids, for example, Japanese Unexamined Patent Publication Nos. 2003-246617 and 03-197318 (1991) and Japanese Examined Patent Publication No. 08-18827 (1996) propose methods for producing meso-holes-bearing calcium carbonate usable for meso-porous materials having uniform meso holes by adding metal ion encapsulating agents to slaked lime solutions and blowing carbonic acid gas to the solutions while adding the metal ion encapsulating agents many times during the reaction and further, for example, Japanese Examined Publication No. 43-3487 discloses a method of adding water glass alone or water glass together with trinitriloacetic acid salt to milk of lime; Japanese Patent Nos. 1140546, 1140542 and 1023550 disclose production of agglomerate and encapsulated and twisted bodies by adding an oxycarboxylic acid, an aminopolycarboxylic acid, a copolymer of isobutylene and maleic acid, or a substance generally called as a chelating agent, which produces a copolymer by coordination with metal ion of a phosphoric acid salt, to milk of lime and blowing carbonic acid gas to the resulting solution for carrying out carbonation reaction to a middle extent so as to produce needle-like, spindle-like or chain-like fine particles and then further carrying out carbonation reaction, so-called vapor phase deposition.
The above-mentioned techniques satisfied or almost satisfied the requirements at that time when they were proposed, however today IT appliances such as mobile phones and personal computers, and FDP such as PDP and LCD have been developed to an extraordinary far extent and the requirements for magnetic recording properties, the electric properties and light diffusibility and the like have become very challenging and it has been highly desired to achieve further improvements.
Even with respect to the films for overlapping wrapping materials, which have been employed in wide application fields where the requirements for the physical properties have not been changed ever before, considerable cost down is required attributed to the recent social phenomena such as so-called price destruction or the like. For cost down, modification and improvements of production process such as saving the production time has been studied and along with today's tendency, the particles to be added are required to be more advantageous and suitable than before for innovative production process.
The above-mentioned precipitated or gel type silica for polyolefin films has been developed while its dispersibility being taken into consideration, however it is still insufficient and thus various problems have been left being unsolved.
For example, when dispersion occurs to an excess extent, films and sheets, which are products, cannot be provided with proper roughness and consequently, they cannot have surfaces with sufficient anti-blocking and slipping properties. Further, a large number of fine particles formed by the excess dispersion may adversely lower the product value, which is deterioration of the transparency and haze of the films.
On the contrary, when the dispersion is insufficient, agglomerates of silica are formed on the films to result in decrease of the product value. Further, dispersibility of silica may differ among respective lots and the dispersion conditions in the production process have to be adjusted. Further, even when same polypropylene resins are used, in the case of using resins with different grades such as melt flow rates or the like, further delicate adjustments are required for production (dispersion) conditions.
Further, since silica has silanol groups in the particle surface, which are water-adsorptive and also since it has a high specific surface area, it contains water in an amount as high as 5 to 7% by weight. In the case of polyolefin films and sheets, for which resins have to be melted inevitably by heating to 150 to 250° C. during the production process involving steps of such as kneading and elongating steps, the water may be foamed to lower the product value and make handling of them difficult and that is not preferable.
Further, in the case silica is used industrially in a large quantity, there may occur many problems, for example, it is required to take careful consideration of the hygienic matter such as silicosis.
The inventions disclosed in Japanese Unexamined Patent Publication Nos. 64-4240 (1989) and 07-196316 (1995) were free from problems of instability of dispersibility, water adsorption, or silicosis which are caused in the case of using silica and they were capable of producing films and sheets with surface roughness of a desired degree and providing the anti-blocking property and the slipping property to the films and the sheets as designed.
Further, with respect to voids caused by drawing in the case of producing polyethylene terephthalate films by biaxial drawing, voids were not at all formed or even when they were formed, that was not so big issue in terms of physical properties or from a technical viewpoint at that time the inventions were accomplished. However, along with the progress of the techniques, thinner films have been required in these years and therefore, high stress has been applied at the time of drawing to result in significant void formation as compared with that by that time and it has become difficult to suppress voids and in some cases, such a phenomenon occurs that particles are broken and fail to provide the slipping property to films.
Further, because of the higher specific gravity of the particles themselves than that of silica, even when particles with the same size are added to films or sheets, the number of the projections in the surfaces is lessened than that in the case of using silica and consequently, that brings disadvantages from an economical viewpoint and therefore, the particles cannot be used for wide use films but only for relatively costly products such as magnetic recording media, capacitors and the like.
With respect to Japanese Unexamined Patent Publication No. 05-4812 (1993), owing to the extremely excellent dispersibility and high affinity with resins, it was studied to find whether the proposal was applicable to biaxially drawn polyethylene terephthalate for base films of capacitors and magnetic recording media, however silanol groups existing in the surfaces of fine silica particles and water adhering to them adversely hindered the resin synthesis, caused a gelling problem and formed foams at the time of resin synthesis and in such a manner, the proposal failed to solve the problems which the silica itself had.
Japanese Patent No. 3260881 proposes particles produced in ethylene glycol and particles produced by drying colloidal calcium carbonate by a spray drier or the like and then sintering the obtained products in carbonic acid gas atmosphere. As the particles produced in ethylene glycol, remarkably fine particles with uniform particle size can be obtained and since they can be obtained in the form of agglomerate of fine particles, it is similar to the case of silica, the invention is expected to compensate the disadvantages of the inventions of Japanese Unexamined Patent Publication Nos. 64-4240 (1989), 07-196316 (1995) and 05-4812 (1993) as well as to have advantageous points.
However, dispersibility owing to the stress applied by kneading and drawing is inferior to that of silica and as byproducts at the time of reaction, coarse particles undetectable by a particle size measuring apparatus are produced in a larger quantity than those produced in the inventions of Japanese Unexamined Patent Publication Nos. 64-4240 (1989) and 07-196316 (1995) and it results in a problem of inferior filtration and thus the issues relevant to voids, the anti-blocking property, the transparency are basically left being unsolved.
Further, in the case ethylene glycol is used for resins other than polyethylene terephthalate which is to be used as a raw material for resins, it is required to dry and evaporate ethylene glycol before adding to films and sheets. However, evaporation of ethylene glycol by drying is more difficult than evaporation of water required in the inventions of Japanese Unexamined Patent Publication Nos. 64-4240 (1989) and 07-196316 (1995) and the particles obtained after the drying do not maintain the similar particle size to that before and even when they are pulverized by dry pulverization, it is difficult to restore the particle size and particle diameter and that results in difficulty of formation of aimed surface roughness in films and sheets.
Incidentally, it is also difficult to use particles obtained by sintering dried bodies of colloidal calcium carbonate in carbonic acid gas atmosphere because of the same reasons described above.
Further, methods described in Japanese Unexamined Patent Publication Nos. 2003-246617 and 03-197318 (1991), Japanese Examined Patent Publication No. 08-18827, Japanese Patent Nos. 41262, 1140546, 1140542, and 1023550 all aim to increase the oil absorption of coated paper materials and the strength by adding particles to rubber and disclose inventions of enlarging the voids of particles, paying attentions to the voids.
However, these inventions are for applications to the coated paper, rubber and the like and although they have a concept relevant to the fineness of the particle size, the inventions naturally lack in adjustment of particle size and particle size distribution which are required for anti-blocking agents and slippage-providing agents for films and sheets and consideration of voids and scratching properties attributed to the stress caused at the time of kneading and drawing.
Accordingly, it has been difficult to use particles and products of these inventions for the anti-blocking agents and slippage-providing agents for films and sheets.